The invention relates to a driver circuit of a liquid crystal display, and more particularly to a driver circuit with three driving voltage levels.
In the active driving technology of liquid crystal display (hereinafter, referred to as an “LCD”), amorphous Si transistors serves as switches for each pixel, wherein the gate of the amorphous Si transistor is coupled to a gate line, the source of the amorphous Si transistor is coupled to a data line, and the drain of the-amorphous Si transistor is coupled to the pixel. When the gate line is at high voltage level, the amorphous Si transistor turns on and thus the pixel obtain the voltage from the data line. When the gate line is at low voltage level, the amorphous Si transistor turns off and the voltage level of the source remains. Because a coupling capacitor Cgs exists between the gate and source, the source voltage level generates a voltage drop, feedthrough voltage ΔVgd, when the voltage of gate changes from high to low voltage level. Thus, the LCD cannot show the correct gray level due to the feedthrough voltage.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of the voltage level of the gate, source and drain of a conventional shift register. During the time period t1, the drain is at a high voltage level VDD, and when the gate is at a high voltage level, the voltage level of the source increases to VDD and then a voltage drop ΔVgd is generated due to the coupling capacitor Cgs. During the time period t2, the drain is at a low voltage level Vss, and when the gate is at a high voltage level, the voltage level of the source decreases to Vss and then a voltage drop ΔVgd is generated due to the coupling capacitor Cgs. According to the conventional shift register of FIG. 1, the TFT transistor cannot stay at a predetermined voltage level due to the coupling capacitor Cgs, and thus, the LCD cannot show the correct gray level.